Lead has been found to have specific and significant effects on chondrocytes, osteoblasts, and osteoclasts in in vitro and animal models at environmentally relevant levels. However, the implications of these effects in humans remains unclear. Preliminary data suggest that lead reduces bone remodeling, with preferential adverse effects on osteoblasts and growth plate chondrocytes. This argues that lead could cause both a decrease in peak bone mass attainment during childhood, as well as increasing the risk of osteoporosis in adults. Furthermore, since bone is the major repository of lead in the body, under circumstances of enhanced bone resorption there is a serious concern that release of lead could result in increased toxicity to other organ systems. This becomes particularly relevant in the case of postmenopausal osteoporosis, where accelerated bone resorption may elevate blood lead levels.
Research on the adverse effects of high lead exposure in humans has primarily focused on neurocognitive outcomes among children. However, a growing body of literature reports that the effects of childhood lead exposure continue into adolescence and adulthood. These effects include delinquent behavior, dental caries, cardiovascular disease, cardiac arrhythmias, and renal dysfunction. These delayed effects are probably due to the fact that lead is harbored in bone for years after initial exposure, and thus may be a source for later exposure.
The skeleton is the major reservoir of lead in the body and consequently its lead content reflects cumulative lead exposure. However, little is known specifically about the effects of lead on the skeleton and the mechanisms of action through which it may affect bone growth and remodeling. In vitro and in vivo experimental data that demonstrate effects on bone and an association between lead exposure and decreased bone density form the basis for the clinical studies in this project, which can examine the relationship between lead exposure and bone mineral density in populations of school-age children, skeletally mature adolescents, and older adults. Examination of the relationship between bone metabolism and cumulative lead exposure in these three distinct age groups representing different phases of bone mass regulation in humans may provide novel insights, and has not been previously undertaken.
Skeletal Effects of Lead Exposure—In Vitro and Animal Studies
Lead has been associated with both low birth weight and short stature, although there is controversy as to whether this results from specific skeletal effects or is related to systemic and nutritional factors. Substantial preliminary data presented in Projects 1, 2, and 3 demonstrate potent specific effects of low levels of lead on chondrocytes, osteoblasts, osteoclasts, and mesenchymal stem cells. In vitro studies have shown that lead has specific effects on growth plate chondrocytes that alter cell phenotype and function. Also, potent inhibitory effects on osteoblast and osteoclast phenotype and function have been demonstrated, suggesting that lead may have effects on bone development and remodeling. Importantly, lead toxicity at environmentally relevant levels has been shown by our group and others to decrease BMD in rats and produce osteoporosis in other animal models. Thus while systemic effects of lead on other organ systems may influence the skeleton, there appear to be direct effects of low levels of lead on the cells involved in bone development, remodeling, and repair. Examination of the histomorphometry of lead intoxicated rats shows abnormalities of the growth plates, including defective remodeling and altered growth plate thickness with loss of proliferating cells and disorganization of the growth plate architecture. Preliminary data in Project 2 demonstrate that lead accelerates growth plate chondrocyte maturation by modulating critical autocrine growth factors. Thus, lead exposure could potentially lead to premature bony maturation.
Skeletal Effects of Lead Exposure—Clinical Studies
Despite the extensive basic-science research on the effects of lead exposure on bone metabolism, human studies on this association are limited. Laraque found that lead-exposed African-American children had a higher mean mineral content than age-matched, white norms. At age 36-47 month, the mean bone mineral content of the radius of the lead-exposed subjects and norms were 0.268 gm/cm2 vs. 0.215 gm/cm2, respectively (P<0.005). The authors attributed the findings to racial differences in bone density. In further analysis, Laraque plotted age against bone density among children with low vs. high lead exposure (i.e., blood lead level ≦29 μg/dL vs. ≧30 μg/dL); the difference in bone density was not significant (p=0.63). Laraque concluded that lead exposure is not associated with changes in bone mineralization. However, since the comparison group was made up of children with moderate-level lead exposure (i.e., blood lead level 12-29 μg/dL), such an analysis cannot exclude the possibility that lead exposure has a threshold effect on bone density at lower blood lead levels. A study found a decreased bone density in rats with a mean blood lead level of only 21 pg/dL when compared to control animals.
Although recent studies report a dramatic decrease in the prevalence of lead exposure in the U.S., this does not imply that adults have not had exposure in the past. As recently as the late 1970s, 78% of the entire U.S. population—children and adults—had blood lead levels ≧10 μg/dL, the current threshold of concern defined by the Centers for Disease Control. This means that the majority of adults in the U.S. had, at some time in the past, an elevated blood lead level, and therefore currently have elevated bone lead levels, given the extremely long half life of lead in bone (τ1/2=20 yr). With this large number of adults who have elevated bone lead levels, and the morbidity associated with osteoporosis, it is important to investigate whether an association exists between lead exposure and osteoporosis in humans.
This research, however, cannot be limited to adults. Although osteoporosis is an affliction of the elderly, its roots may be established during childhood. An individual who does not achieve peak bone mass during childhood may be at-risk for osteoporosis in later life. Since 90% or more of bone mass is achieved by age 17-20 years, the association between lead exposure and bone density should be examined among children and skeletally mature adolescents.
Bone Mass and Osteoporosis
Osteoporosis is a disease of epidemic proportions in the United States, with an estimated 25 million affected individuals. The health care costs of treatment of osteoporotic hip and spine fractures and associated morbidity may exceed $10 billion per year. During childhood and adolescence, bone mass accretion occurs through both the activities of the chondrocytes in the growth plates and the bone forming activity of osteoblasts. Thus, bone mass and bone mineral density (BMD) gradually increase during growth. Bone mass is a reflection of the relative rates of bone formation and bone resorption, and continues to increase into early adulthood after growth plate closure, until peak bone mass is reached at age 25-30. After peak bone mass is reached, bone resorption rates exceed formation rates, and there is a gradual loss of bone with further aging in both men and women. At menopause, the loss of estrogen results in an acceleration of the rate of bone loss, which occurs over approximately a 10-year period, subsequently slowing to a rate resembling premenopausal bone loss rates. Systemic stimuli of bone resorption, such as corticosteroids, thyroid or parathyroid disease, can further enhance the rate of bone loss. BMD correlates with bone strength and inversely with fracture risk. As bone is lost, an individual can cross a theoretical “fracture threshold” where the BMD and associated bone strength are insufficient to resist nontraumatic forces on the bone and can lead to fractures. Men achieve greater peak bone mass levels and are not subject to the accelerated decline associated with menopause, so bone density levels are less likely to cross the fracture threshold, explaining the 5-6-fold higher incidence of osteoporosis in women as compared to men. Also there are other genetic determinants of bone mass such as race. Black men and women have been shown to have higher peak bone mass compared with white and Hispanic individuals, thus accounting for the decreased incidence of osteoporosis among blacks. Antiresorptive treatments such as estrogen, estrogen analogs, and bisphosphonates at menopause have been shown to prevent accelerated bone loss and reduce fracture risk in perimenopausal women. The inhibitory effects of lead on chondrocytes and osteoblasts could result in attainment of a lower peak bone mass, predisposing lead-exposed individuals to osteoporosis later in life. Furthermore, lead released from bone matrix during increased resorption, such as at menopause, could increase local concentrations at bone surfaces, worsening the inhibition of osteoblastic bone formation, and therefore accelerating the loss of bone by further disturbing the balance of bone formation and resorption. Osteoporosis associated with lead intoxication has been previously reported, consistent with our own findings and those of others with animal models of lead toxicity causing osteopenia.
Systemic Toxicities of Skeletal Lead
Because bone has been shown to be the repository for 90-95% of lead in the body, there has been significant concern that release of lead from bone during menopause could lead to elevated blood levels and significant toxicity. A study of blood lead levels in Hispanic women demonstrated higher blood leads in women within the first 4 years of menopause as compared to women who were more than 4 years postmenopausal, supporting this concern. Similarly, data from the National Health and Nutrition Examination Survey II (NHANES II, 1976-1980) demonstrated a significant increase in blood leads following menopause. Further support for this observation was presented in a prior study of antiresorptive therapy with estrogen in lead-exposed women by Webber et al (1995). He showed that bone lead was significantly higher in women treated with estrogen than in the untreated group. More potent antiresorptive agents such the bisphosphonates, alendronate or risedronate, which are now standard therapies for postmenopausal osteoporosis, have not been studied with regard to effects on lead metabolism.
Numerous other conditions of accelerated bone resorption have been shown to increase blood lead levels, including immobilization, thyrotoxicosis, and hyperparathyroidism. The significance of the accelerated release of lead from bone in postmenopausal women relates to the known toxicities of low levels of lead on other organ systems, such as the central nervous and cardiovascular systems. Lead intoxication at low levels has been associated with hypertension in both children and adults, and has been shown to cause cognitive dysfunction in older adults. Elevated lead levels have been reported after immobilization for a fracture, with associated renal toxicity in one report. This has added clinical significance given data that demonstrate lead-related abnormalities of fracture healing. Local elevation of extracellular fluid lead levels at a fracture site secondary to increased resorption from the combined inflammatory response and immobilization could impair or delay the healing of osteoporotic fractures. The data on systemic toxicities of bone-derived lead form the basis for the proposed clinical trial of alendronate in lead-exposed menopausal women to determine if a potent antiresorptive can prevent the rise of blood lead during accelerated bone resorption.
Measurement of Bone Lead
The method of K shell X-ray Fluorescence (KXRF) has been employed to measure bone lead in occupationally lead exposed and unexposed individuals. In vivo bone lead measurements were first developed by Ahlgren and Mattsson at Lund in Sweden. They used X-rays from a 57Co source to excite lead K shell electrons. Measurement was made in a finger bone and this system has been in operation since 1972. Subsequently, an improved system was developed by Chettle, Scott, Laird and Somervaille in Birmingham, England. This system used X-rays from a 109Cd source to excite the lead K electrons to produce gamma rays (i.e., X-ray fluorescence), and proved to have three particular advantages over the original approach: measurements were more precise; bone lead content could be directly related to bone mineral; and measurements could usefully be made of any superficial bone. The first human measurements were made in 1983 and initially the tibia lead concentration was studied. Since then the calcaneus has frequently been selected as a trabecular bone site. The patella has also been used in this way, and measurements have also been reported of radius, sternum and skull. This measurement approach has been adopted by a number of laboratories around the world. In 1991, Chettle and Webber at McMaster University developed a new system, with improved precision, based on the same principles.
Two particular features of the relationship between bone lead and lead exposure have consistently emerged from studies in which K X-ray fluorescence technology has been employed. First, bone lead concentration reflects cumulative lead exposure. Cumulative exposure can be represented by the time-weighted integral of blood lead, monitored regularly in lead exposed workers. Secondly, release of lead from bone contributes to circulating lead in blood, thus constituting an endogenous exposure. This relationship is particularly clear when industrial exposure has ceased. For such people, endogenous exposure can often be the dominant contributor to current blood lead.